Fun with double and variable stars

City stargazers have a distinct disadvantage when it comes to viewing bright nebulae and galaxies, but they are on equal ground with their country counterparts when it comes to observing double and variable stars. Rather than spread their weak light out across a disk, stars concentrate their full energy output into a pinpoint beam that punches through light pollution and other interferences far more effectively than deep-sky "faint fuzzies." In this installment, we look at each of these fascinating families of objects.

At least half of the stars in the universe — some studies suggest as many as 85 percent — are not just single points of light, but instead, belong to double or multiple star systems. In some cases, a dwarf star is revolving around a giant sun. In others, two stars of equal size orbit a common point in between the two, while in other cases still, several stars orbit one another. Regardless of the actual number involved, any stellar system containing two or more stars is referred to generically as a double star or binary star.

We can break this down further into four broad categories. Double stars that can be resolved through telescopes are referred to as visual binaries. No two pairs appear exactly the same. Many shine pure white, while others flicker with distinctive colors. Most appear in pairs, although in some cases, as many as six or seven stars belong to a single system.

Some binary stars are so close to each other that the only way astronomers can tell if the star is single or double is through spectral analysis. These are known as spectroscopic binaries. Still others are detectable when the secondary star's orbit carries it in front of, and then behind, the primary, causing the system's total light output to fluctuate. Theses are called eclipsing binaries. Then, there are the posers — two unrelated stars that just happen to lie along the same line of sight from Earth. These are referred to as optical doubles.

Double starsThe brightest star in a binary system is designated as the "A" star and is often referred to as the system's primary. The fainter companion star is dubbed "B." Usually, the distinction between the two is obvious, but, in some systems with equal-magnitude components, it can be difficult to tell which is which. Adding to the mix, if there are still more stars involved, they will be assigned letters in alphabetical order, such as "C," "D," and so on.

In reality, the component stars may be hundreds or thousands of astronomical units apart, but their apparent distance, or how far apart they appear from our vantage point on Earth, is usually measured in seconds of arc. One arcsecond (written as 1") is one-sixtieth of an arcminute (1'), and 60 arcminutes make up a degree.

Just how close a pair your telescope or binoculars can resolve depends on several factors, such as atmospheric steadiness, the instrument's optical quality and cleanliness, your personal vision, and how disparate the component stars' magnitudes are. That last factor is critically important because glare from a bright primary star can easily overwhelm the feeble light from a dim companion. One of the best examples of this situation is the star Sirius. Because of Sirius A's overwhelming brilliance, spotting its companion white dwarf Sirius B, or the Pup as it's nicknamed, is one of the toughest tests around for a telescope, even under perfect conditions.

Okay, it's time to crack open the sky's jewelry store. Here are 40 of my favorite double and multiple stars  10 for each season. Each seasonal sky holds many beautiful binary-star systems. Many display striking contrasts in magnitude, while others are nearly equal in brightness. Some of these gems shine pure white, while others glimmer with distinctive shades of blue, yellow, red, and orange.

On the next clear night, go outside with your telescope or binoculars and try to find as many of the double stars on this list as possible. First, adjust your telescope's magnification until you see star images that are crisp and clear. Use just enough magnification to split the stars. Start with no more than 50x to find the star. Once it's in view, take a look. Can you split it? If not, increase the magnification to 75x or 100x and take another look. That should be more than enough magnification to resolve each of these; so, if you still can't, then Earth's turbulent atmosphere might be blurring the view. Move on to the next target, and come back the next clear night to any you missed.

Spring's double stars

Object

Constellation

R.A.*

Dec.*

Mag*.

Sep.*

Recommended magnification1

Epsilon (ε)

Boötes

14h45.0m

27° 04'

2.5,4.9

3"

100x

Xi (Ξ)

Boötes

14h51.4m

19° 06'

4.7, 7.0

7"

50x

Iota Ι

Cancer

8h46.7m

28° 46'

4.2, 6.6

30"

15x

Zeta (Ζ)

Cancer

8h12.2m

17° 39'

5.6, 6.0

6"

50x

Cor Caroli (α)

Canes Venatici

12h56.0m

38° 19'

2.9, 5.5

19"

25x

Alpha (α) Centauri

Centaurus

14h39.6m

-60° 50'

0.0, 1.4

14"

25x

Acrux (Α)

Crux

12h26.6m

-63° 06'

1.4, 1.9

4"

100x

Gamma (γ)

Leo

10h20.0m

19° 51'

2.2, 3.5

4"

100x

Mizar (Ζ)

Ursa Major

13h23.9m

54° 56'

2.3, 4.0

14"

25x

Porrima (Γ)

Virgo

12h41.7m

-01° 27'

3.5, 3.5

4"

100x

Note 1: Stars with recommended magnifications between 7x and 15x are best viewed through binoculars.

Don't just glance at each of these stars and move on. Instead, study each target carefully. Can you see any color in the stars? You may miss it if you look too quickly. If not, try defocusing the star images just slightly and try again. Sometimes, blurring the stars will make subtle colors a little more obvious. Start a sketchbook of what you see. Try to record the view as accurately as possible, and always include other stars in field as well as the targeted double. If any of the stars show color, be sure to note it in writing. Better still, use a set of colored artist pencils to draw the view.

Variable starsLike double stars, variable stars are not as adversely affected by light pollution as some other sky targets are. In fact, some of the most dedicated variable-star observers conduct their studies right in the hearts of major cities. They head out nearly every clear night to watch "their" stars, to estimate magnitudes and record the findings for later submission to research associations.

There are three major classes of variable stars, with each group broken down into several sub-groups. The first, eclipsing binaries, I mentioned earlier. It takes Algol about 3 days to complete a cycle from a high of magnitude 2.1 to a low of 3.4, and back again.

Pulsating variable stars actually expand and contract, like a beating heart. Some pulse rhythmically, while others flicker irregularly, suffering from the stellar equivalent of cardiac fibrillation, while still others vary on a semi-regular basis caused by two or more overlapping periods. Some, such as Cepheid variables (named for the genre's prototypical star, Delta [δ] Cephei), take only hours to complete a cycle. Others, called long-period variables, can have periods in excess of one year.

Eruptive variable stars are the most dramatic of all. Eruptive variables usually lie low, only to change brightness suddenly and unpredictably in just a few days, hours, or even minutes. The best known example of an eruptive variable is a nova, which can intensify by 5 or more magnitudes in less than a day. Other eruptive variables, like R Coronae Borealis, shine near their maximum magnitudes, only to drop precipitously as they exhale a cloud of obscuring carbon soot that blocks some of their light.

Regardless of type, variable stars are always cataloged by constellation using capital Roman letters. The first variable to be discovered in a constellation is dubbed "R," the second "S," then "T," and so on. For example, R Coronae Borealis was the first variable found in Corona Borealis, while S Scuti would have been the second discovered in Scutum, and so on. Once the ninth variable was cataloged in a particular constellation, denoted by "Z," the next was identified by "RR," followed by "RS" to "RZ," then "SS" to "SZ," and continuing in this fashion until "ZZ" was discovered. After this, "AA" to "AZ" are assigned, then "BB" to "BZ," etc. The system continues, excluding "JJ" to "JZ," until "QZ" is used.

Using this confusing system, astronomers can catalog 334 variable stars in a single constellation. This might seem to be adequate to cover all variables in a constellation, but some along the main stream of the Milky Way contain even more. In these cases, the 335th variable identified in a particular constellation is listed as "V335" (V for variable), with each subsequent discovery placed in numerical order.

Many amateur astronomers enjoy monitoring variable stars. Andy Beaton is a dedicated variable star observer from Toronto. "In spite of the severe handicaps with my location," he says, "I have a list of 60 variable stars that I monitor regularly. I try to observe 5 to 10 of them every clear night." Astronomy columnist Glenn Chaple also observes variable stars from his yard in the Boston suburbs every chance he gets. He is closing in on a lifetime total of 70,000 magnitude estimates  an achievement exceeded by only a handful of other observers, regardless of location.

Amateur observations of variable stars also constitute a very important source of data toward our understanding the universe. The American Association of Variable Star Observers (AAVSO) is an international organization devoted to the study of these stars. Through systematic monitoring, members keep a careful watch over variable-star activity and report magnitude estimates to AAVSO headquarters in Cambridge, Massachusetts. The AAVSO acts as a liaison between amateur astronomers contributing data and professional astronomers requesting data.

So, not only can you observe variable stars from a city, you can also contribute valuable scientific observations. Best of all, estimating the magnitude of a variable star is not as difficult as it may sound. The AAVSO has a wide variety of detailed charts available for selected variables. Each chart shows the targeted star and its immediate surroundings.

Below's a sample chart showing Z Ursae Majoris. Z is a great star for beginners because its circumpolar position means it is above the horizon throughout the year for most stargazers in the Northern Hemisphere. It is also bright enough to remain visible in small telescopes throughout its magnitude cycle.

Take a look at the chart. The variable is smack-dab in the middle, represented by a small circle. Notice that many of the nearby stars have numbers next to them &#151; those numbers are the apparent magnitudes, to the nearest tenth of a magnitude, of the indicated stars. The decimals points are removed to avoid confusing them for another star. For example, the star marked "59" at the top of the chart is magnitude 5.9.

CES

You might not believe it's possible, but amateurs can successfully estimate the unknown magnitude of a variable to an accuracy of 0.1 magnitude. By comparing the variable to two stars of known fixed brightness, one brighter and one fainter, you can make an accurate reading. Let's say that when you find Z, it appears brighter than the nearby magnitude 8.0 (labeled 80) star, but fainter than the magnitude 7.2 (72) star. Next, check it against the magnitude 7.6 (76) comparison star. If Z appears fainter than 7.6, then that means it is magnitude 7.7, 7.8, or 7.9. Is it closer to the 7.6 star or the 8.0 star? If it seems closer to the 8.0 star than to the 7.6 star, then your estimate would be 7.9; if Z appears exactly halfway in between, then your estimate would be 7.8, and so on.

Becoming an accomplished variable-star observer is a skill that takes time to cultivate. Rather than try to estimate a large number of stars, restrict your selection to one or two. Again, Z UMa, as it is often abbreviated, is a good first variable. Become familiar with finding it, and get to know its surroundings. Not only will this help you find it quickly each time you return, it will also give you a chance to recognize the comparison stars.

Record your first estimate, and then wait. Come back in a week or so and take a look at it again. What is its magnitude this time? Has it gone up or down? Record your second estimate, and then wait another week. Repeat the process over the course of a month, and see if there is a pattern to the estimates. If they seem regular and rhythmic, then check them against recent observations that have been posted on the AAVSO web site. How do yours compare? Are they close and, more importantly, do they match the star's behavior as observed by others? In other words, do your magnitude estimates move in the same direction as those submitted by more experienced observers? Or are they erratic, sometimes going up, then down, and then up again? If so, then continue to practice.

Here are few tips to improve your technique. First, always base your estimates on at least two comparison stars, one dimmer than the variable and one brighter. This is called "bracketing." The closer the comparison stars are to the variable's brightness, the better. Let's say that a variable you're viewing is fainter than a 7.3-magnitude star, but brighter than a 7.9-magnitude star. Is there another comparison star between those two values? If so, use it as a check to help narrow down the possibilities. If that third comparison star is, for example, magnitude 7.6 and it also appears fainter than the variable, then your estimate would be either magnitude 7.4 or 7.5.

Always keep the variable and any comparison stars near the center of the field. Many eyepieces suffer from vignetting, which causes stars to dim near the edge. That can cause a comparison star to appear fainter than it actually is. If need be, use a lower-power eyepiece to expand the field and keep everything in view.

Also, be aware that the human eye has a few idiosyncrasies that, under certain circumstances, can cause red stars to appear brighter than they really are. This can lead to errors when observing long-period variables, which are usually red giant stars. One, called the Purkinje effect, causes red stars to grow in brightness the longer you stare straight at them. That's why it's important to keep your eye moving back and forth between the variable and any comparison stars. Next, red stars may also look brighter than comparison stars depending on how they are oriented in the field of view. If a red star is oriented above or below a white comparison star, the red star will appear brighter than if the two are oriented directly left or right of each other. Finally, a bright sky, due to light pollution, twilight, or moonlight, can also make red stars appear unnaturally bright.

In addition to Z UMa, here are a few other variable stars to add to your program.

As your confidence grows, consider joining the AAVSO. Before you know it, not only will you become a talented observer, you will also be contributing scientifically valuable observations that will help pave the way for professional astronomers to study these enigmatic stars. And just imagine, you're actually doing this from an observing site that you may have thought to be unusable at first!

As Rod Mollise, author of The Urban Astronomer's Guide (Springer, 2006), puts it, "Never sell the sky short. If you can see any stars at all, start looking; you may be surprised at the wonders it offers up."